Horde Mode/Survival Mode......whatever you wanna call it. Basically, you have two teams: the Horde and the Warders.
The Horde is a big group of monsters(or whatever) trying to to take over this Plane.
To do so, they have to kill the Warders.
The Horde gets unlimited respawns, but the Warders only get X number of respawns.
For the Horde to win, they must kill all of the Defenders within Y number of minutes.
If they don't, the Warders have a mage (who isn't actually seen, we assume he's safe inside the castle/fort/etc) who closes the portal to the Horde's home Plane and the Horde defeated.
This game can incorporate a fort, like with the cabin battle.
If you can think of anything else, lemme know.

Re: Horde Mode/Survival Mode...Sounds like closing the portal won't kill the invaders. That job might be left to the remaining Warders.

With this in mind, it would be necessary to tightly tweak the X number of respawns vis-a-vis the Y number of minutes. I'd suggest a fairly short game.

If Y is short, it encourages the invaders to rampage (and die a lot) in order to kill the Warders' numbers. Once the time limit is up, the remainder of the invaders would have to fight smart to out-live the remainder of the Warders.